Many communication systems utilize a modulation format in which a digital data signal modulates the amplitude of a time varying waveform or carrier signal. To increase the information-carrying capacity of the system, this modulation process is often performed simultaneously using two digital data signals wherein one data signal modulates the amplitude of a carrier signal, and the other data signal modulates the amplitude of a phase quadrature carrier signal of the same frequency. This modulation of phase quadrature carrier signals is referred to by a variety of names, such as quadrature amplitude modulation (QAM), phase shift keying (PSK), or amplitude and phase shift keying (APSK). The information represented by the digital data signal is, of course, virtually limitless and can represent voice, video, facsimile and the like. Moreover, the transmission channel propagating the modulated carriers is also not limited and, at present, may include air, wire or lightguide.
In communication systems which modulate the amplitudes of quadrature-related carrier signals, each data signal is converted into a mutli-level signal before being spectrally shaped into a continuous waveform by Nyquist filtering. Each of these continuous waveforms is then supplied to a multiplier for modulating the amplitude of an associated one of the quadrature-related carrier signals. Finally, the modulated carriers are then summed into one signal which is transmitted or further processed prior to transmission. One problem with the above described signal processing is that it is typically provided by analog circuitry which is expensive and not readily adaptable to design changes. For example, changing the baud rate can require time consuming redesign of the Nyquist filter. Another disadvantage is that the described circuit implementations cannot use baseband signal shaping to compensate for cross-rail distortions generated by the signal processing or transmission medium. Still another disadvantage is that time-consuming adjustment of an oscillator network can be required to generate the quadrature-related carrier signals.
In one recent development, described in a U.S. patent application to W. Debus, Jr. et al, entitled "Digital Synthesis Technique for Pulses Having Predetermined Time and Frequency Domain Characteristics", Ser. No. 517,803, filed July 27, 1983 and assigned to the present assignee, some of the prior art limitations are eliminated. As disclosed, a memory based technique is used to provide spectral shaping of the data signals thereby eliminating the analog Nyquist filters. In addition, the values stored in the memory can be adjusted so as to compensate for in-rail distortion. This improvement, however, still requires the need for oscillator based circuitry to generate the quadrature-related carriers, multipliers to perform the modulation of the carriers, and an adder to sum the modulated carrier signals. In addition, the disclosed technique cannot compensate for cross-rail distortions. Accordingly, the digital synthesis of the amplitude modulation of a carrier signal with spectrally shaped data signals which can provide in-rail and cross-rail distortion compensation would be desirable.